1. Field of the Invention
The present invention relates to computer input/output data transfer system, and more particularly to an input/output data transfer system which is capable of transferring data between channel devices and input/output devices over transmission lines having different transfer capacity bandwidths in a multiplexed or divided data form for enhancing the utilization rate of the transmission lines and permitting selection of various data transfer rates.
2. Description of Related Art
Recently, there is a trend in the art of mainframe computer systems, away from a parallel input/output interface for transferring data in parallel over a plurality of copper wire lines, toward a serial input/output interface which provides a higher capacity of data transfer and longer distance connections over optical fiber cables. The serial input/output interfaces prevalent at present are capable of transferring data over a distance on the order of tens of kilometers by means of fiber links having a data transfer capacity of 200 megabits/second and repeaters including switching devices.
With advances in optical transmission technology, the channel capacity of data transfer and transmission distance have increased significantly. Today, it is rather difficult to clearly distinguish a channel input/output interface from a local area network (LAN) using techniques such as a token ring, FDDI (Fiber Distributed Data Interface), ATM (Asynchronous Transfer Mode), etc. In current mainframe computer system architecture, there coexist various kinds of channel devices. Examples include an open-network direct-access channel arrangement allowing data transfer over a distance on the order of hundreds of kilometers, an optical fiber open-connection channel interface allowing high capacity data transfer at a rate of gigabits/second, a system interlinking interface for data transfer between computer systems, etc. Therefore, there is a high demand for higher-speed and longer-distance transmission capabilities in an input/output interface for connecting input/output devices including external storage devices with a host computer.
In an arrangement based on a conventional technique related to channels in the above-mentioned serial input/output interface, for example, a switched point-to-point topology using 200-Mbps optical fiber links and switching devices is employed to provide star-networking connections of channels and input/output devices. In this configuration in which all the links have the same capacity of data transfer, a connection is established between nodes at both ends so that a maximum data transfer capacity of each transfer line can be used in input/output operations.
Since costs of components operating at a rate of gigabits/second have decreased substantially in recent years, there is an increasing demand for efficient, smooth expansion of conventional input/output interface facilities to improve input/output processing performance meeting enhanced system performance. At present, it is desirable to provide a technique for implementing a large-capacity interface for higher performance of input/output operations in a novel manner different from such conventional performance improvement methods that ensure input/output throughput by increasing the number of channels, developing new high-performance channel interfaces, etc.
With respect to enhancing input/output operation speed, increasing the data transfer capacity per link is more advantageous since the ratio of input/output operation time in a certain period of actual data transfer is made larger thereby, i.e., since the amount of data to be transferred per input/output operation is made larger. However, the command instruction processing time and the end-of-execution report processing time required before and after data transfer are not reduced substantially, and the answer waiting time increases where a transmission distance of an interface is longer. In a situation where the amount of data to be handled per input/output operation is rather small, an advantageous effect corresponding to an increase in data transfer capacity per link cannot be attained with conventional input/output interface protocols.
A conventional technique for the efficient use of data transfer capacity per link is found in Japanese Non-examined Patent Publication No. 187277/1994, for example. In this conventional technique, link sharing by means of frame multiplexing is performed for input/output devices which can operate in parallel simultaneously on the same link. In another conventional technique of this kind, packets are multiplexed by means of frame multiplexing in data transfer through a plurality of networked nodes, or in another technique, interfaces of links are multiplexed virtually or actually by means of time-division/frequency-division multiplexing. Still more, there is another conventional data transfer protocol technique intended for increasing data transfer speed by means of simplifying overhead processing before and after data transfer.
At present, futuristic input/output host computer systems including new input/output interfaces and protocols thereof are under development. However, it will take a period of more than several years to ten years to establish the technology for realizing such futuristic input/output systems including input/output devices. In consideration of reliability, compatibility with upper-level applications, etc. as well as input/output performance, it is impracticle to rebuild entire computer systems over and over again to meet rapid, continuous advances in optical transmission technology. Therefore, stepwise improvements in performance, which are different from those accomplished by ordinary advancements in system architecture expansion, are important and needed at present in a CPU and input/output interface of host computer systems.
For enhancement in computer system performance, it is increasingly in demand to improve input/output processing performance together with CPU performance. In most of the recent mainframe computer systems having high-performance channel capabilities, the number of accommodated channels is increasing appreciably, i.e., 256, 512 or more channels are connected to each computer system.
An optical fiber serial input/output interface cable is far lighter in weight and easier to handle than a copper-wire parallel input/output interface cable. However, the increase in numbers of channels and diversification in interface route arrangement using switching devices, as well as the installation and maintenance of input/output interface cabling, give rise to considerable problems.
For simplifying installation and maintenance of input/output interface cabling, a method of bundling interface cables by means of trunk cables, etc. has been devised. However, this method is disadvantageous in that its application to existing facilities located across floors in a building or across buildings is rather difficult. A method of replacement with the latest-technology input/output interface capable of high-speed large-capacity data transfer is also disadvantageous in that input/output devices including channel devices must be modified or replaced with new ones.